The present invention relates to a sub-method of a prior known method for CO2 capture such as plants described as part of the prior art in the applicants own patent application WO 2008/06379 A (AKER CLEAN CARBON AS) May 29, 2008.
Different absorbents are used and/or proposed in the process to absorb CO2. The selection of the absorbent will typically be based upon energy cost, construction cost, and emission limit. The absorbents are typically aqueous solutions of one or more CO2 absorbing chemicals. Typical CO2 absorbing chemicals used for the currently used CO2 absorbents include amines and carbonates. Typical amines are MEA, DEA, AMP, MDEA, MMEA, AEP and piperazine. Many of these absorbents degrade slowly during the absorption process. Amine solutions degrade by reaction with oxygen, carbon dioxide, sulphur compounds, nitrous oxides and other impurities found in the exhaust gas. The reaction products include heat stable salts and other degradation products. Degradation of the amine results in a reduced active amine concentration and hence reduces the efficiency of the process. Continuous make-up of amine may be required to replace degraded amine.
In addition to the CO2 absorbing chemicals, the absorbent may include different other chemical compounds. The additional chemical compounds may include de-foamers, corrosion inhibitors etc.
The concentration of degrading products in the amine solution should be restricted. Degrading products are often corrosive and reduce the efficiency of the system. Typically a maximum concentration of degradation products is set between 2% and 10%.
The process of separating impurities, degraded solvent and heat stable salts from the absorbent that may be cycled into the absorption/desorption cycle is often referred to as reclaiming of the absorbent.
There are several methods for removing degradation products from the amine system. Typically a side stream is withdrawn from the amine system and sent to a purification system. The purification system removes some of the degradation products and the purified amine is returned to the amine system. These systems can operate continuously, semi-continuously or batch. Typical methods and technology used for purification include thermal reclaiming, adsorption, ion exchange, membranes and electro dialysis.
In a thermal reclaiming system a fraction, typically 0.5 to 5%, of the lean and hot absorbent leaving the regenerator is bled off and heated further in a reclaimer. Alkali is preferably injected into the reclaimer. The alkali is added to liberate and recover molecular amine from the heat stable salts. In the reclaimer the absorbent is heated further to evaporate the amine together with steam. The evaporated amine and steam is withdrawn and introduced into the stripper, or regeneration column, in the absorption desorption cycle of the capture plant.
The reclaimer is often a kettle type unit and is typically heated by steam. Normally, the reclaimer operates at the same pressure as the stripper so that vapours generated in the reclaimer can be fed directly to the stripper. Considerable heat is required by the reclaimer, however this heat is not lost from the system as the vapour are returned to the stripper. High boiling point components and salts build up in the reclaimer. As the concentration of these components increase, the boiling point of the mixture increases and hence the operating temperature of the reclaimer increases. The fluid in the reclaimer is removed in a drain and sent to disposal.
EP 1967250 A (MITSUBISHI HEAVY INDUSTRIES, S, LTD) Jan. 17, 2008 relates to a CO2 recovery system and a method for removing solid particles for use in the system. The method for removal of solid particles comprises a filtering unit for removal of solid particle from the circulating lean solvent. The retentat from the filter(s) is removed from the filter(s) by backwashing with backwash water. The backwash water including the particles removed from the filters is then introduced into an evaporator where the dispersion of particles in the backwash water is heated to evaporate any free amine and steam that are introduced into the regeneration column as stripping gas, and the remaining more concentrated suspension in the evaporator is withdrawn and introduced into a reclaimer where the particles are further concentrated by heating and generation of steam that is withdrawn and introduced into the regeneration column as stripping gas. The further concentrated suspension is removed from the reclaimer to be deposited. There is no indication in EP1967250 on flashing of lean absorbent or lowering of the pressure in the reclaimer to improve reclaiming. The question of the need to reduce the pressure and thus the temperature in the reclaimer is not discussed.
Thermal reclaiming at the stripper operating pressure is no option for amines having a high boiling temperature. The temperatures required to reclaim these amines would result in very high degradation rates within the reclaimer. Operating the reclaimer at high temperature will result in scaling of the heat transfer area, increased amine degradation in the reclaimer and possible production of gaseous decomposition products that are returned to the stripper.
It is therefore advantageous to operate the reclaimer at lower temperatures. The most common method of reducing the reclaimer operating temperature is to reduce the reclaimer operating pressure. This is often called vacuum reclaiming. An additional advantage with low temperature reclaiming is that a lower temperature heating medium can be used. Low temperature heating mediums usually have a lower cost than higher temperature heating mediums.
U.S. Pat. No. 5,389,208 A (CANADIAN CHEMICAL RECLAIMING, LTD) Feb. 14, 1995 describes a process for reclaiming and/or concentration of aqueous solutions of chemicals, such as alkanolamines used for removal of acid components in natural gas. A combination of temperature and vacuum is used to evaporate water and/or alkanolamine for recycling to the absorption process.
Low pressure or vacuum reclaiming requires the introduction of a compressor system or vacuum pump system. Vacuum pump systems involve condensing most of the vapour and therefore are not energy efficient. Using a compressor will reduce the energy consumption considerably, however the capital cost associated with a dedicated compressor normally excludes this option.
Accordingly, there is a need for a method and a device for vacuum or low pressure reclaiming having a reduced energy requirement compared to use of a vacuum pump but where the capital cost of the solution is lower than installing a dedicated compressor.